Produced by Robert J. Hall




[Illustration: Fig. 1. The Constellation of Orion (Hubble).

Photographed with a small camera lens of 1 inch aperture and 5
inches focal length. The three bright stars in the centre of the
picture form the belt of Orion. Just below, in the sword handle, is
an irregular white patch about one-eighth of an inch in diameter.
This is a small-scale image of the great nebula in Orion, shown
on a larger scale in Fig. 2.]




THE NEW HEAVENS


BY

GEORGE ELLERY HALE

DIRECTOR OF THE MOUNT WILSON OBSERVATORY OF THE CARNEGIE INSTITUTION
OF WASHINGTON

WITH NUMEROUS ILLUSTRATIONS


NEW YORK

CHARLES SCRIBNER'S SONS

1922




TO MY WIFE




PREFACE

Fourteen years ago, in a book entitled "The Study of Stellar Evolution"
(University of Chicago Press, 1908), I attempted to give in untechnical
language an account of some modern methods of astrophysical research.
This book is now out of print, and the rapid progress of science has
left it completely out of date. As I have found no opportunity to
prepare a new edition, or to write another book of similar purpose,
I have adopted the simpler expedient of contributing occasional
articles on recent developments to _Scribner's Magazine_, three
of which are included in the present volume.

I am chiefly indebted, for the illustrations, to the Mount Wilson
Observatory and the present and former members of its staff whose
names appear in the captions. Special thanks are due to Mr. Ferdinand
Ellerman, who made all of the photographs of the observatory buildings
and instruments, and prepared all material for reproduction. The
cut of the original Cavendish apparatus is copied from the
_Philosophical Transactions for 1798_ with the kind permission
of the Royal Society, and I am also indebted to the Royal Society
and to Professor Fowler and Father Cortie for the privilege of
reproducing from the _Proceedings_ two illustrations of their
spectroscopic results.

G. E. H.

January, 1922.




CONTENTS

CHAPTER
    I.  THE NEW HEAVENS
   II.  GIANT STARS
  III.  COSMIC CRUCIBLES



ILLUSTRATIONS

FIG.
  1. The Constellation of Orion (Hubble)
  2. The Great Nebula in Orion (Pease)
  3. Model by Ellerman of summit of Mount Wilson, showing the observatory
     buildings among the trees and bushes
  4. The 100-inch Hooker telescope
  5. Erecting the polar axis of the 100-inch telescope
  6. Lowest section of tube of 100-inch telescope, ready to leave Pasadena
     for Mount Wilson
  7. Section of a steel girder for dome covering the 100-inch telescope,
     on its way up Mount Wilson
  8. Erecting the steel building and revolving dome that cover the Hooker
     telescope
  9. Building and revolving dome, 100 feet in diameter, covering the
     100-inch Hooker telescope
 10. One-hundred-inch mirror, just silvered, rising out of the
     silvering-room in pier before attachment to lower end of telescope
     tube. (Seen above)
 11. The driving-clock and worm-gear that cause the 100-inch Hooker
     telescope to follow the stars
 12. Large irregular nebula and star cluster in Sagittarius (Duncan)
 13. Faint spiral nebula in the constellation of the Hunting Dogs (Pease)
 14. Spiral nebula in Andromeda, seen edge on (Ritchey)
 15. Photograph of the moon made on September 15, 1919, with the 100-inch
     Hooker telescope (Pease)
 16. Photograph of the moon made on September 15, 1919, with the 100-inch
     Hooker telescope (Pease)
 17. Hubble's Variable Nebula. One of the few nebulæ known to vary in
     brightness and form
 18. Ring Nebula in Lyra, photographed with the 60-inch (Ritchey) and
     100-inch (Duncan) telescopes
 19. Gaseous prominence at the sun's limb, 140,000 miles high (Ellerman)
 20. The sun, 865,000 miles in diameter, from a direct photograph showing
     many sun-spots (Whitney)
 21. Great sun-spot group, August 8, 1917 (Whitney)
 22. Photograph of the hydrogen atmosphere of the sun (Ellerman)
 23. Diagram showing outline of the 100-inch Hooker telescope, and path of
     the two pencils of light from a star when under observation with the
     20-foot Michelson interferometer
 24. Twenty-foot Michelson interferometer for measuring star diameters,
     attached to upper end of the skeleton tube of the 100-inch Hooker
     telescope
 25. The giant Betelgeuse (within the circle), familiar as the conspicuous
     red star in the right shoulder of Orion (Hubble)
 26. Arcturus (within the white circle), known to the Arabs as the "Lance
     Bearer," and to the Chinese as the "Great Horn" or the "Palace of the
     Emperors" (Hubble)
 27. The giant star Antares (within the white circle), notable for its red
     color in the constellation Scorpio, and named by the Greeks "A Rival
     of Mars" (Hubble)
 28. Diameters of the Sun, Arcturus, Betelgeuse, and Antares compared with
     the orbit of Mars
 29. Aldebaran, the "leader" (of the Pleiades), was also known to the Arabs
     as "The Eye of the Bull," "The Heart of the Bull," and "The Great
     Camel" (Hubble)
 30. Solar prominences, photographed with the spectroheliograph without an
     eclipse (Ellerman)
 31. The 150-foot tower telescope of the Mount Wilson Observatory
 32. Pasadena Laboratory of the Mount Wilson Observatory
 33. Sun-spot vortex in the upper hydrogen atmosphere (Benioff)
 34. Splitting of spectrum lines by a magnetic field (Bacock)
 35. Electric furnace in the Pasadena Laboratory of the Mount Wilson
     Observatory
 36. Titanium oxide in red stars
 37. Titanium oxide in sun-spots
 38. The Cavendish experiment
 39. The Trifid Nebula in Sagittarius (Ritchey)
 40. Spiral nebula in Ursa Major (Ritchey)
 41. Mount San Antonio as seen from Mount Wilson




CHAPTER I

THE NEW HEAVENS

Go out under the open sky, on a clear and moon-less night, and try
to count the stars. If your station lies well beyond the glare of
cities, which is often strong enough to conceal all but the brighter
objects, you will find the task a difficult one. Ranging through
the six magnitudes of the Greek astronomers, from the brilliant
Sirius to the faintest perceptible points of light, the stars are
scattered in great profusion over the celestial vault. Their number
seems limitless, yet actual count will show that the eye has been
deceived. In a survey of the entire heavens, from pole to pole,
it would not be possible to detect more than from six to seven
thousand stars with the naked eye. From a single viewpoint, even
with the keenest vision, only two or three thousand can be seen.
So many of these are at the limit of visibility that Ptolemy's
"Almagest," a catalogue of all the stars whose places were measured
with the simple instruments of the Greek astronomers, contains
only 1,022 stars.

Back of Ptolemy, through the speculations of the Greek philosophers,
the mysteries of the Egyptian sun-god, and the observations of the
ancient Chaldeans, the rich and varied traditions of astronomy stretch
far away into a shadowy past. All peoples, in the first stirrings
of their intellectual youth, drawn by the nightly splendor of the
skies and the ceaseless motions of the planets, have set up some
system of the heavens, in which the sense of wonder and the desire
for knowledge were no less concerned than the practical necessities
of life. The measurement of time and the needs of navigation have
always stimulated astronomical research, but the intellectual demand
has been keen from the first. Hipparchus and the Greek astronomers
of the Alexandrian school, shaking off the vagaries of magic and
divination, placed astronomy on a scientific basis, though the
reaction of the Middle Ages caused even such a great astronomer
as Tycho Brahe himself to revert for a time to the practice of
astrology.

EARLY INSTRUMENTS

The transparent sky of Egypt, rarely obscured by clouds, greatly
favored Ptolemy's observations. Here was prepared his great star
catalogue, based upon the earlier observations of Hipparchus, and
destined to remain alone in its field for more than twelve centuries,
until Ulugh Bey, Prince of Samarcand, repeated the work of his
Greek predecessor. Throughout this period the stars were looked
upon mainly as points of reference for the observation of planetary
motions, and the instruments of observation underwent little change.
The astrolabe, which consists of a circle divided into degrees,
with a rotating diametral arm for sighting purposes, embodies their
essential principle. In its simple form, the astrolabe was suspended
in a vertical plane, and the stars were observed by bringing the
sights on the movable diameter to bear upon them. Their altitude
was then read off on the circle. Ultimately, the circle of the
astrolabe, mounted with one of its diameters parallel to the earth's
axis, became the armillary sphere, the precursor of our modern
equatorial telescope. Great stone quadrants fixed in the meridian
were also employed from very early times. Out of such furnishings,
little modified by the lapse of centuries, was provided the elaborate
instrumental equipment of Uranibourg, the great observatory built
by Tycho Brahe on the Danish island of Huen in 1576. In this "City
of the Heavens," still dependent solely upon the unaided eye as a
collector of starlight, Tycho made those invaluable observations
that enabled Kepler to deduce the true laws of planetary motion. But
after all these centuries the sidereal world embraced no objects,
barring an occasional comet or temporary star, that lay beyond
the vision of the earliest astronomers. The conceptions of the
stellar universe, except those that ignored the solid ground of
observation, were limited by the small aperture of the human eye.
But the dawn of another age was at hand.

[Illustration: Fig. 2. The Great Nebula in Orion (Pease).

Photographed with the 100-inch telescope. This short-exposure photograph
shows only the bright central part of the nebula. A longer exposure
reveals a vast outlying region.]

The dominance of the sun as the central body of the solar system,
recognized by Aristarchus of Samos nearly three centuries before
the Christian era, but subsequently denied under the authority of
Ptolemy and the teachings of the Church, was reaffirmed by the
Polish monk Copernicus in 1543. Kepler's laws of the motions of the
planets, showing them to revolve in ellipses instead of circles,
removed the last defect of the Copernican system, and left no room for
its rejection. But both the world and the Church clung to tradition,
and some visible demonstration was urgently needed. This was supplied
by Galileo through his invention of the telescope.

[Illustration: Fig. 3. Model by Ellerman of summit of Mount Wilson,
showing the observatory buildings among the trees and bushes.

The 60-foot tower on the extreme left, which is at the edge of
a precipitous cañon 1,500 feet deep, is the vertical telescope
of the Smithsonian Astrophysical Observatory. Above it are the
"Monastery" and other buildings used as quarters by the astronomers
of the Mount Wilson Observatory while at work on the mountain. (The
offices, computing-rooms, laboratories, and shops are in Pasadena.)
Following the ridge, we come successively to the dome of the 10-inch
photographic telescope, the power-house, laboratory, Snow horizontal
telescope, 60-foot-tower telescope, and 150-foot-tower telescope,
these last three used for the study of the sun. The dome of the
60-inch reflecting telescope is just below the 150-foot tower,
while that of the 100-inch telescope is farther to the right. The
altitude of Mount Wilson is about 5,900 feet.]

The crystalline lens of the human eye, limited by the iris to a
maximum opening about one-quarter of an inch in diameter, was the
only collector of starlight available to the Greek and Arabian
astronomers. Galileo's telescope, which in 1610 suddenly pushed
out the boundaries of the known stellar universe and brought many
thousands of stars into range, had a lens about 2-1/4 inches in
diameter. The area of this lens, proportional to the square of
its diameter, was about eighty-one times that of the pupil of the
eye. This great increase in the amount of light collected should
bring to view stars down to magnitude 10.5, of which nearly half
a million are known to exist.

It is not too much to say that Galileo's telescope revolutionized
human thought. Turned to the moon, it revealed mountains, plains,
and valleys, while the sun, previously supposed immaculate in its
perfection, was seen to be blemished with dark spots changing from
day to day. Jupiter, shown to be accompanied by four encircling
satellites, afforded a picture in miniature of the solar system,
and strongly supported the Copernican view of its organization,
which was conclusively demonstrated by Galileo's discovery of the
changing phases of Venus and the variation of its apparent diameter
during its revolution about the sun. Galileo's proof of the Copernican
theory marked the downfall of mediævalism and established astronomy on
a firm foundation. But while his telescope multiplied a hundredfold
the number of visible stars, more than a century elapsed before
the true possibilities of sidereal astronomy were perceived.

[Illustration: Fig. 4. The 100-inch Hooker telescope.]

STRUCTURE OF THE UNIVERSE

Sir William Herschel was the first astronomer to make a serious
attack upon the problem of the structure of the stellar universe.
In his first memoir on the "Construction of the Heavens," read
before the Royal Society in 1784, he wrote as follows:

"Hitherto the sidereal heavens have, not inadequately for the purpose
designed, been represented by the concave surface of a sphere in
the centre of which the eye of an observer might be supposed to be
placed.... In future we shall look upon those regions into which we
may now penetrate by means of such large telescopes, as a naturalist
regards a rich extent of ground or chain of mountains containing
strata variously inclined and directed as well as consisting of
very different materials."

On turning his 18-inch reflecting telescope to a part of the Milky
Way in Orion, he found its whitish appearance to be completely
resolved into small stars, not separately seen with his former
telescopes. "The glorious multitude of stars of all possible sizes
that presented themselves here to my view are truly astonishing; but
as the dazzling brightness of glittering stars may easily mislead
us so far as to estimate their number greater than it really is,
I endeavored to ascertain this point by counting many fields, and
computing from a mean of them, what a certain given portion of
the Milky Way might contain." By this means, applied not only to
the Milky Way but to all parts of the heavens, Herschel determined
the approximate number and distribution of all the stars within
reach of his instrument.

By comparing many hundred gauges or counts of stars visible in
a field of about one-quarter of the area of the moon, Herschel
found that the average number of stars increased toward the great
circle which most nearly conforms with the course of the Milky Way.
Ninety degrees from this plane, at the pole of the Milky Way, only
four stars, on the average, were seen in the field of the telescope.
In approaching the Milky Way this number increased slowly at first,
and then more and more rapidly, until it rose to an average of
122 stars per field.

[Illustration: Fig. 5. Erecting the polar axis of the 100-inch
telescope.]

These observations were made in the northern hemisphere, and
subsequently Sir John Herschel, using his father's telescope at
the Cape of Good Hope, found an almost exactly similar increase
of apparent star density for the southern hemisphere. According to
his estimates, the total number of stars in both hemispheres that
could be seen distinctly enough to be counted in this telescope
would probably be about five and one-half millions.

The Herschels concluded that "the stars of our firmament, instead
of being scattered in all directions indifferently through space,
form a stratum of which the thickness is small, in comparison with
its length and breadth; and in which the earth occupies a place
somewhere about the middle of its thickness, between the point
where it subdivides into two principal laminæ inclined at a small
angle to each other." This view does not differ essentially from our
modern conception of the form of the Galaxy; but as the Herschels
were unable to see stars fainter than the fifteenth magnitude,
it is evident that their conclusions apply only to a restricted
region surrounding the solar system, in the midst of the enormously
extended sidereal universe which modern instruments have brought
within our range.

MODERN METHODS

The remarkable progress of modern astronomy is mainly due to two
great instrumental advances: the rise and development of the
photographic telescope, and the application of the spectroscope to
the study of celestial objects. These new and powerful instruments,
supplemented by many accessories which have completely revolutionized
observatory equipment, have not only revealed a vastly greater
number of stars and nebulæ: they have also rendered feasible
observations of a type formerly regarded as impossible. The chemical
analysis of a faint star is now so easy that it can be accomplished
in a very short time--as quickly, in fact, as an equally complex
substance can be analyzed in the laboratory. The spectroscope also
measures a star's velocity, the pressure at different levels in
its atmosphere, its approximate temperature, and now, by a new
and ingenious method, its distance from the earth. It determines
the velocity of rotation of the sun and of nebulæ, the existence
and periods of orbital revolution of binary stars too close to
be separated by any telescope, the presence of magnetic fields
in sunspots, and the fact that the entire sun, like the earth, is
a magnet.

[Illustration: Fig. 6. Lowest section of tube of 100-inch telescope,
ready to leave Pasadena for Mount Wilson.]

Such new possibilities, with many others resulting from the application
of physical methods of the most diverse character, have greatly
enlarged the astronomer's outlook. He may now attack two great
problems: (1) The structure of the universe and the motions of
its constituent bodies, and (2) the evolution of the stars: their
nature, origin, growth, and decline. These two problems are intimately
related and must be studied as one.[*]

[Footnote *: A third great problem open to the astronomer, the
study of the constitution of matter, is described in Chapter III.]

If space permitted, it would be interesting to survey the progress
already accomplished by modern methods of astronomical research.
Hundreds of millions of stars have been photographed, and the boundaries
of the stellar universe have been pushed far into space, but have not
been attained. Globular star clusters, containing tens of thousands
of stars, are on so great a scale (according to Shapley) that light,
travelling at the rate of 186,000 miles per second, may take 500
years to cross one of them, while the most distant of these objects
may be more than 200,000 light-years from the earth. The spiral
nebulæ, more than a million in number, are vast whirling masses
in process of development, but we are not yet certain whether they
should be regarded as "island universes" or as subordinate to the
stellar system which includes our minute group of sun and planets,
the great star clouds of the Milky Way, and the distant globular
star clusters.

[Illustration: Fig. 7. Section of a steel girder for dome covering
the 100-inch telescope, on its way up Mount Wilson.]

These few particulars may give a slight conception of the scale
of the known universe, but a word must be added regarding some
of its most striking phenomena. The great majority of the stars
whose motions have been determined belong to one or the other of
two great star streams, but the part played by these streams in the
sidereal system as a whole is still obscure. The stars have been
grouped in classes, presumably in the order of their evolutional
development, as they pass from the early state of gaseous masses, of
low density, through the successive stages resulting from loss of
heat by radiation and increased density due to shrinkage. Strangely
enough, their velocities in space show a corresponding change,
increasing as they grow older or perhaps depending upon their mass.

It is impossible within these limits to do more than to give some
indication of the scope of the new astronomy. Enough has been said,
however, to assist in appreciating the increased opportunity for
investigation, and the nature of the heavy demands made upon the
modern observatory. But before passing on to describe one of the
latest additions to the astronomer's instrumental equipment, a
word should be added regarding the chief classes of telescopes.

REFRACTORS AND REFLECTORS

Astronomical telescopes are of two types: refractors and reflectors.
A refracting telescope consists of an object-glass composed of
two or more lenses, mounted at the upper end of a tube, which is
pointed at the celestial object. The light, after passing through
the lenses, is brought to a focus at the lower end of the tube, where
the image is examined visually with an eyepiece, or photographed
upon a sensitive plate. The largest instruments of this type are
the 36-inch Lick telescope and the 40-inch refractor of the Yerkes
Observatory.

[Illustration: Fig. 8. Erecting the steel building and revolving
dome that cover the Hooker telescope.]

Reflecting telescopes, which are particularly adapted for photographic
work, though also excellent for visual observations, are very
differently constructed. No lens is used. The telescope tube is
usually built in skeleton form, open at its upper end, and with
a large concave mirror supported at its base. This mirror serves
in place of a lens. Its upper surface is paraboloidal in shape,
as a spherical surface will not unite in a sharp focus the rays
coming from a distant object. The light passes through no glass--a
great advantage, especially for photography, as the absorption
in lenses cuts out much of the blue and violet light, to which
photographic plates are most sensitive. The reflection occurs on
the _upper_ surface of the mirror, which is covered with a coat of
pure silver, renewed several times a year and always kept highly
burnished. Silvered glass is better than metals or other substances
for telescope mirrors, chiefly because of the perfection with which
glass can be ground and polished, and the ease of renewing its
silvered surface when tarnished.

The great reflectors of Herschel and Lord Rosse, which were provided
with mirrors of speculum metal, were far inferior to much smaller
telescopes of the present day. With these instruments the star images
were watched as they were carried through the field of view by the
earth's rotation, or kept roughly in place by moving the telescope
with ropes or chains. Photographic plates, which reveal invisible
stars and nebulæ when exposed for hours in modern instruments, were
not then available. In any case they could not have been used,
in the absence of the perfect mechanism required to keep the star
images accurately fixed in place upon the sensitive film.

[Illustration: Fig. 9. Building and revolving dome, 100 feet in
diameter, covering the 100-inch Hooker telescope.

Photographed from the summit of the 150-foot-tower telescope.]

It would be interesting to trace the long contest for supremacy
between refracting and reflecting telescopes, each of which, at
certain stages in its development, appeared to be unrivalled. In
modern observatories both types are used, each for the purpose for
which it is best adapted. For the photography of nebulæ and the
study of the fainter stars, the reflector has special advantages,
illustrated by the work of such instruments as the Crossley and Mills
reflectors of the Lick Observatory; the great 72-inch reflector,
recently brought into effective service at the Dominion Observatory
in Canada; and the 60-inch and 100-inch reflectors of the Mount
Wilson Observatory.

The unaided eye, with an available area of one-twentieth of a square
inch, permits us to see stars of the sixth magnitude. Herschel's
18-inch reflector, with an area 5,000 times as great, rendered
visible stars of the fifteenth magnitude. The 60-inch reflector,
with an area 57,600 times that of the eye, reveals stars of the
eighteenth magnitude, while to reach stars of about the twentieth
magnitude, photographic exposures of four or five hours suffice
with this instrument.

Every gain of a magnitude means a great gain in the number of stars
rendered visible. Stars of the second magnitude are 3.4 times as
numerous as those of the first, those of the eighth magnitude are
three times as numerous as those of the seventh, while the sixteenth
magnitude stars are only 1.7 as numerous as those of the fifteenth
magnitude. This steadily decreasing ratio is probably due to an
actual thinning out of the stars toward the boundaries of the stellar
universe, as the most exhaustive tests have failed to give any
evidence of absorption of light in its passage through space. But
in spite of this decrease, the gain of a single additional magnitude
may mean the addition of many millions of stars to the total of those
already shown by the 60-inch reflector. Here is one of the chief
sources of interest in the possibilities of a 100-inch reflecting
telescope.

100-INCH TELESCOPE

[Illustration: Fig. 10. One-hundred-inch mirror, just silvered,
rising out of the silvering-room in pier before attachment to lower
end of telescope tube. (Seen above.)]

In 1906 the late John D. Hooker, of Los Angeles, gave the Carnegie
Institution of Washington a sum sufficient to construct a telescope
mirror 100 inches in diameter, and thus large enough to collect
160,000 times the light received by the eye. (Fig. 10.) The casting
and annealing of a suitable glass disk, 101 inches in diameter
and 13 inches thick, weighing four and one-half tons, was a most
difficult operation, finally accomplished by a great French glass
company at their factory in the Forest of St. Gobain. A special
optical laboratory was erected at the Pasadena headquarters of
the Mount Wilson Observatory, and here the long task of grinding,
figuring, and testing the mirror was successfully carried out by
the observatory opticians. This operation, which is one of great
delicacy, required years for its completion. Meanwhile the building,
dome, and mounting for the telescope were designed by members of
the observatory staff, and the working drawings were prepared. An
opportune addition by Mr. Carnegie to the endowment of the Carnegie
Institution of Washington, of which the observatory is a branch,
permitted the necessary appropriations to be made for the completion
and erection of the telescope. Though delayed by the war, during
which the mechanical and optical facilities of the observatory
shops were utilized for military and naval purposes, the telescope
is now in regular use on Mount Wilson.

The instrument is mounted on a massive pier of reinforced concrete,
33 feet high and 52 feet in diameter at the top. A solid wall extends
south from this pier a distance of 50 feet, on the west side of
which a very powerful spectrograph, for photographing the spectra
of the brightest stars, will be mounted. Within the pier are a
photographic dark room, a room for silvering the large mirror (which
can be lowered into the pier), and the clock-room, where stands
the powerful driving-clock, with which the telescope is caused
to follow the apparent motion of the stars. (Fig. 11.)

[Illustration: Fig. 11. The driving-clock and worm-gear that cause
the 100-inch Hooker telescope to follow the stars.]

The telescope mounting is of the English type, in which the telescope
tube is supported by the declination trunnions between the arms of
the polar axis, built in the form of a rectangular yoke carried by
bearings on massive pedestals to the north and south. These bearings
must be aligned exactly parallel to the axis of the earth, and must
support the polar axis so freely that it can be rotated with perfect
precision by the driving-clock, which turns a worm-wheel 17 feet in
diameter, clamped to the lower end of the axis. As this motion
must be sufficiently uniform to counteract exactly the rotation
of the earth on its axis, and thus to maintain the star images
accurately in position in the field of view, the greatest care
had to be taken in the construction of the driving-clock and in
the spacing and cutting of the teeth in the large worm-wheel. Here,
as in the case of all of the more refined parts of the instrument,
the work was done by skilled machinists in the observatory shops in
Pasadena or on Mount Wilson after the assembling of the telescope.
The massive sections of the instrument, some of which weigh as
much as ten tons each, were constructed at Quincy, Mass., where
machinery sufficiently large to build battleships was available.
They were then shipped to California, and transported to the summit
of Mount Wilson over a road built for this purpose by the construction
division of the observatory, which also built the pier on which the
telescope stands, and erected the steel building and dome that
cover it.

[Illustration: Fig. 12. Large irregular nebula and star cluster
in Sagittarius (Duncan).

Photographed with the 60-inch telescope.]

[Illustration: Fig. 13. Faint spiral nebula in the constellation
of the Hunting Dogs (Pease).

Photographed with the 60-inch telescope.]

The parts of the telescope which are moved by the driving-clock
weigh about 100 tons, and it was necessary to provide means of
reducing the great friction on the bearings of the polar axis. To
accomplish this, large hollow steel cylinders, floating in mercury
held in cast-iron tanks, were provided at the upper and lower ends
of the polar axis. Almost the entire weight of the instrument is
thus floated in mercury, and in this way the friction is so greatly
reduced that the driving-clock moves the instrument with perfect
ease and smoothness.

The 100-inch mirror rests at the bottom of the telescope tube on
a special support system, so designed as to prevent any bending of
the glass under its own weight. Electric motors, forty in number, are
provided to move the telescope rapidly or slowly in right ascension
(east or west) and in declination (north or south), for focussing
the mirrors, and for many other purposes. They are also used for
rotating the dome, 100 feet in diameter, under which the telescope
is mounted, and for opening the shutter, 20 feet wide, through
which the observations are made.

A telescope of this kind can be used in several different ways.
The 100-inch mirror has a focal length of about 42 feet, and in
one of the arrangements of the instrument, the photographic plate
is mounted at the centre of the telescope tube near its upper end,
where it receives directly the image formed by the large mirror. In
another arrangement, a silvered glass mirror, with plane surface,
is supported near the upper end of the tube at an angle of 45°, so
as to form the image at the side of the tube, where the photographic
plate can be placed. In this case, the observer stands on a platform,
which is moved up and down by electric motors in front of the opening
in the dome through which the observations are made.

[Illustration: Fig. 14. Spiral nebula in Andromeda, seen edge on
(Ritchey).

Photographed with the 60-inch telescope.]

Other arrangements of the telescope, for which auxiliary convex
mirrors carried near the upper end of the tube are required, permit
the image to be photographed at the side of the tube near its lower
end, either with or without a spectrograph; or with a very powerful
spectrograph mounted within a constant-temperature chamber south
of the telescope pier. In this last case, the light of a star is
so reflected by auxiliary mirrors that it passes down through a
hole in the south end of the polar axis and brings the star to
a focus on the slit of the fixed spectrograph.

ATMOSPHERIC LIMITATIONS

The huge dimensions of such a powerful engine of research as the
Hooker telescope are not in themselves a source of satisfaction to
the astronomer, for they involve a decided increase in the labor
of observation and entail very heavy expense, justifiable only in
case important results, beyond the reach of other instruments,
can be secured. The construction of a telescope of these dimensions
was necessarily an experiment, for it was by no means certain, after
the optical and mechanical difficulties had been overcome, that
even the favorable atmosphere of California would be sufficiently
tranquil to permit sharply defined celestial images to be obtained
with so large an aperture. It is therefore important to learn what
the telescope will actually accomplish under customary observing
conditions.

Fortunately we are able to measure the performance of the instrument
with certainty. Close beside it on Mount Wilson stands the 60-inch
reflector, of similar type, erected in 1908. The two telescopes can
thus be rigorously compared under identical atmospheric conditions.

The large mirror of the 100-inch telescope has an area about 2.8
times that of the 60-inch, and therefore receives nearly three times
as much light from a star. Under atmospheric conditions perfect
enough to allow all of this light to be concentrated in a point,
it should be capable of recording on a photographic plate, with a
given exposure, stars about one magnitude fainter than the faintest
stars within reach of the 60-inch. The increased focal length,
permitting such objects as the moon to be photographed on a larger
scale, should also reveal smaller details of structure and render
possible higher accuracy of measurement. Finally, the greater
theoretical resolving power of the larger aperture, providing it
can be utilized, should permit the separation of the members of
close double stars beyond the range of the smaller instrument.

CRITICAL TESTS

The many tests already made indicate that the advantages expected
of the new telescope will be realized in practice. The increased
light-gathering power will mean the addition of many millions of
stars to those already known. Spectroscopic observations now in
regular progress have carried the range of these investigations
far beyond the possibilities of the 60-inch telescope. A great
class of red stars, for example, almost all the members of which
were inaccessible to the 60-inch, are now being made the subject
of special study. And in other fields of research equal advantages
have been gained.

The increase in the scale of the images over those given by the
60-inch telescope is illustrated by two photographs of the Ring
Nebula in Lyra, reproduced in Fig. 18. The Great Nebula in Orion,
photographed with the 100-inch telescope with a comparatively short
exposure, sufficient to bring out the brighter regions, is reproduced
in Fig. 2. It is interesting to compare this picture with the
small-scale image of the same nebula shown in Fig. 1.

[Illustration: Fig. 15. Photograph of the moon made on September
15, 1919, with the 100-inch Hooker telescope (Pease).

The ring-like formations are the so-called craters, most of them
far larger than anything similar on the earth. That in the lower
left corner with an isolated mountain in the centre is Albategnius,
sixty-four miles in diameter. Peaks in the ring rise to a height
of fifteen thousand feet above the central plain. Note the long
sunset shadows cast by the mountains on the left. The level region
below on the right is an extensive plain, the Mare Nubium.]

[Illustration: Fig. 16. Photograph of the moon made on September
15, 1919, with the 100-inch Hooker telescope (Pease).

The mountains above and to the left are the lunar Apennines; those
on the left just below the centre are the Alps. Both ranges include
peaks from fifteen thousand to twenty thousand feet in height. In
the upper right corner is Copernicus, about fifty miles in diameter.
The largest of the conspicuous group of three just below the Apennines
is Archimedes and at the lower end of the Alps is Plato. Note the
long sunset shadows cast by the isolated peaks on the left. The
central portion of the picture is a vast plain, the Mare Imbrium.]

The sharpness of the images given by the new telescope may be
illustrated by some recent photographs of the moon, obtained with
an equivalent focal length of 134 feet. In Fig. 15 is shown a rugged
region of the moon, containing many ring-like mountains or craters.
Fig. 16 shows the great arc of the lunar Apennines (above) and the
Alps (below), to the left of the broad plain of the Mare Imbrium.
The starlike points along the moon's terminator, which separates
the dark area from the region upon which the sun (on the right)
shines, are the mountain peaks, about to disappear at sunset. The
long shadows cast by the mountains just within the illuminated
area are plainly seen. Some of the peaks of the lunar Apennines
attain a height of 20,000 feet.

In less powerful telescopes the stars at the centre of the great
globular clusters are so closely crowded together that they cannot
be studied separately with the spectrograph. Moreover, most of
them are much too faint for examination with this instrument. At
the 134-foot focus the 100-inch telescope gives a large-scale image
of such clusters, and permits the spectra of stars as faint as
the fifteenth magnitude to be separately photographed.

[Illustration: Fig. 17. Hubble's Variable Nebula. One of the few
nebulæ known to vary in brightness and form.

Photographed with the 100-inch telescope (Hubble).]

CLOSE DOUBLE STARS

A remarkable use of the 100-inch telescope, which permits its full
theoretical resolving power to be not merely attained but to be
doubled, has been made possible by the first application of Michelson's
interference method to the measurement of very close double stars.
When employing this, the 100-inch mirror is completely covered,
except for two slits. Beams of light from a star, entering by the
slits, unite at the focus of the telescope, where the image is
examined by an eyepiece magnifying about five thousand diameters.
Across the enlarged star image a series of fine, sharp fringes is
seen, even when the atmospheric conditions are poor. If the star is
single the fringes remain visible, whatever the distance between the
slits. But in the case of a star like Capella, previously inferred
to be double from the periodic displacement of the lines in its
spectrum, but with components too close together to be distinguished
separately, the fringes behave differently. As the slits are moved
apart a point is reached where the fringes completely disappear,
only to reappear as the separation is continued. This effect is
obtained when the slits are at right angles to the line joining
the two stars of the pair, found by this method to be 0.0418 of a
second of arc apart (on December 30, 1919). Subsequent measures,
of far greater precision than those obtainable by other methods in
the case of easily separated double stars, show the rapid orbital
motion of the components of the system. This device will be applied
to other close binaries, hitherto beyond the reach of measurement.

[Illustration: Fig. 18. Ring Nebula in Lyra, photographed with the
60-inch (Ritchey) and 100-inch (Duncan) telescopes.

Showing the increased scale of the images given by the larger
instrument.]

Without entering into further details of the tests, it is evident
that the new telescope will afford boundless possibilities for
the study of the stellar universe.[*] The structure and extent of
the galactic system, and the motions of the stars comprising it;
the distribution, distances, and dimensions of the spiral nebulæ,
their motions, rotation, and mode of development; the origin of
the stars and the successive stages in their life history: these
are some of the great questions which the new telescope must help
to answer. In such an embarrassment of riches the chief difficulty
is to withstand the temptation toward scattering of effort, and to
form an observing programme directed toward the solution of crucial
problems rather than the accumulation of vast stores of miscellaneous
data. This programme will be supplemented by an extensive study
of the sun, the only star near enough the earth to be examined
in detail, and by a series of laboratory investigations involving
the experimental imitation of solar and stellar conditions, thus
aiding in the interpretation of celestial phenomena.

[Footnote *: It is not adapted for work on the sun, as the mirrors
would be distorted by its heat. Three other telescopes, especially
designed for solar observations, are in use on Mount Wilson.]




CHAPTER II

GIANT STARS

Our ancestral sun, as pictured by Laplace, originally extended
in a state of luminous vapor beyond the boundaries of the solar
system. Rotating upon its axis, it slowly contracted through loss
of heat by radiation, leaving behind it portions of its mass, which
condensed to form the planets. Still gaseous, though now denser than
water, it continues to pour out the heat on which our existence
depends, as it shrinks imperceptibly toward its ultimate condition
of a cold and darkened globe.

Laplace's hypothesis has been subjected in recent years to much
criticism, and there is good reason to doubt whether his description
of the mode of evolution of our solar system is correct in every
particular. All critics agree, however, that the sun was once enormously
larger than it now is, and that the planets originally formed part
of its distended mass.

Even in its present diminished state, the sun is huge beyond easy
conception. Our own earth, though so minute a fragment of the primeval
sun, is nevertheless so large that some parts of its surface have
not yet been explored. Seen beside the sun, by an observer on one
of the planets, the earth would appear as an insignificant speck,
which could be swallowed with ease by the whirling vortex of a
sun-spot. If the sun were hollow, with the earth at its centre,
the moon, though 240,000 miles from us, would have room and to
spare in which to describe its orbit, for the sun is 865,000 miles
in diameter, so that its volume is more than a million times that
of the earth.

[Illustration: Fig. 19. Gaseous prominence at the sun's limb, 140,000
miles high (Ellerman).

Photographed with the spectroheliograph, using the light emitted
by glowing calcium vapor. The comparative size of the earth is
indicated by the white circle.]

But what of the stars, proved by the spectroscope to be self-luminous,
intensely hot, and formed of the same chemical elements that constitute
the sun and the earth? Are they comparable in size with the sun? Do
they occur in all stages of development, from infancy to old age?
And if such stages can be detected, do they afford indications
of the gradual diminution in volume which Laplace imagined the
sun to experience?

[Illustration: Fig. 20. The sun, 865,000 miles in diameter, from
a direct photograph showing many sun-spots (Whitney)

The small black disk in the centre represents the comparative size
of the earth, while the circle surrounding it corresponds in diameter
to the orbit of the moon.]

STAR IMAGES

Prior to the application of the powerful new engine of research
described in this article we have had no means of measuring the
diameters of the stars. We have measured their distances and their
motions, determined their chemical composition, and obtained undeniable
evidence of progressive development, but even in the most powerful
telescopes their images are so minute that they appear as points
rather than as disks. In fact, the larger the telescope and the
more perfect the atmospheric conditions at the observer's command,
the smaller do these images appear. On the photographic plate, it is
true, the stars are recorded as measurable disks, but these are due
to the spreading of the light from their bright point-like images,
and their diameters increase as the exposure time is prolonged.
From the images of the brighter stars rays of light project in
straight lines, but these also are instrumental phenomena, due
to diffraction of light by the steel bars that support the small
mirror in the tube of reflecting telescopes. In a word, the stars
are so remote that the largest and most perfect telescopes show
them only as extremely minute needle-points of light, without any
trace of their true disks.

[Illustration: Fig. 21. Great sun-spot group, August 8, 1917 (Whitney).

The disk in the corner represents the comparative size of the earth.]

How, then, may we hope to measure their diameters? By using, as
the man of science must so often do, indirect means when the direct
attack fails. Most of the remarkable progress of astronomy during
the last quarter-century has resulted from the application of new and
ingenious devices borrowed from the physicist. These have multiplied
to such a degree that some of our observatories are literally physical
laboratories, in which the sun and stars are examined by powerful
spectroscopes and other optical instruments that have recently advanced
our knowledge of physics by leaps and bounds. In the present case
we are indebted for our star-measuring device to the distinguished
physicist Professor Albert A. Michelson, who has contributed a long
array of novel apparatus and methods to physics and astronomy.

THE INTERFEROMETER

The instrument in question, known as the interferometer, had previously
yielded a remarkable series of results when applied in its various
forms to the solution of fundamental problems. To mention only a
few of those that have helped to establish Michelson's fame, we may
recall that our exact knowledge of the length of the international
metre at Sevres, the world's standard of measurement, was obtained
by him with an interferometer in terms of the invariable length of
light-waves. A different form of interferometer has more recently
enabled him to measure the minute tides within the solid body of the
earth--not the great tides of the ocean, but the slight deformations
of the earth's body, which is as rigid as steel, that are caused by
the varying attractions of the sun and moon. Finally, to mention
only one more case, it was the Michelson-Morley experiment, made
years ago with still another form of interferometer, that yielded
the basic idea from which the theory of relativity was developed
by Lorentz and Einstein.

[Illustration: Fig. 22. Photograph of the hydrogen atmosphere of
the sun (Ellerman).

Made with the spectroheliograph, showing the immense vortices,
or whirling storms like tornadoes, that centre in sun-spots. The
comparative size of the earth is shown by the white circle traced
on the largest sun-spot.]

The history of the method of measuring star diameters is a very
curious one, showing how the most promising opportunities for scientific
progress may lie unused for decades. The fundamental principle
of the device was first suggested by the great French physicist
Fizeau in 1868. In 1874 the theory was developed by the French
astronomer Stéphan, who observed interference fringes given by a
large number of stars, and rightly concluded that their angular
diameters must be much smaller than 0.158 of a second of arc, the
smallest measurable with his instrument. In 1890 Michelson, unaware
of the earlier work, published in the _Philosophical Magazine_ a
complete description of an interferometer capable of determining
with surprising accuracy the distance between the components of
double stars so close together that no telescope can separate them.
He also showed how the same principle could be applied to the
measurement of star diameters if a sufficiently large interferometer
could be built for this purpose, and developed the theory much
more completely than Stéphan had done. A year later he measured
the diameters of Jupiter's satellites by this means at the Lick
Observatory. But nearly thirty years elapsed before the next step
was taken. Two causes have doubtless contributed to this delay. Both
theory and experiment have demonstrated the extreme sensitiveness
of the "interference fringes," on the observation of which the
method depends, and it was generally supposed by astronomers that
disturbances in the earth's atmosphere would prevent them from
being clearly seen with large telescopes. Furthermore, a very large
interferometer, too large to be carried by any existing telescope,
was required for the star-diameter work, though close double stars
could have been easily studied by this device with several of the
large telescopes of the early nineties. But whatever the reasons,
a powerful method of research lay unused.

The approaching completion of the 100-inch telescope of the Mount
Wilson Observatory led me to suggest to Professor Michelson, before
the United States entered the war, that the method be thoroughly
tested under the favorable atmospheric conditions of Southern
California. He was at that time at work on a special form of
interferometer, designed to determine whether atmospheric disturbances
could be disregarded in planning large-scale experiments. But the
war intervened, and all of our efforts were concentrated for two
years on the solution of war problems.[*] In 1919, as soon as the
100-inch telescope had been completed and tested, the work was
resumed on Mount Wilson.

[Footnote *: Professor Michelson's most important contribution during
the war period was a new and very efficient form of range-finder,
adopted for use by the U. S. Navy.]

A LABORATORY EXPERIMENT

The principle of the method can be most readily seen by the aid
of an experiment which any one can easily perform for himself with
simple apparatus. Make a narrow slit, a few thousandths of an inch
in width, in a sheet of black paper, and support it vertically
before a brilliant source of light. Observe this from a distance of
40 or 50 feet with a small telescope magnifying about 30 diameters.
The object-glass of the telescope should be covered with an opaque
cap, pierced by two circular holes about one-eighth of an inch in
diameter and half an inch apart. The holes should be on opposite
sides of the centre of the object-glass and equidistant from it,
and the line joining the holes should be horizontal. When this
cap is removed the slit appears as a narrow vertical band with
much fainter bands on both sides of it. With the cap in place, the
central bright band appears to be ruled with narrow vertical lines
or fringes produced by the "interference"[*] of the two pencils of
light coming through different parts of the object-glass from the
distant slit. Cover one of the holes, and the fringes instantly
disappear. Their production requires the joint effect of the two
light-pencils.

[Footnote *: For an explanation of the phenomena of interference,
see any encyclopæedia or book on physics.]

Now suppose the two holes over the object-glass to be in movable
plates, so that their distance apart can be varied. As they are
gradually separated the narrow vertical fringes become less and
less distinct, and finally vanish completely. Measure the distance
between the holes and divide this by the wavelength of light, which
we may call 1/50000 of an inch. The result is the angular width
of the distant slit. Knowing the distance of the slit, we can at
once calculate its linear width. If for the slit we substitute a
minute circular hole, the method of measurement remains the same,
but the angular diameter as calculated above must be multiplied
by 1.22.[*]

[Footnote *: More complete details may be found in Michelson's Lowell
Lectures on "Light-Waves and Their Uses," University of Chicago
Press, 1907.]

To measure the diameter of a star we proceed in a similar way,
but, as the angle it subtends is so small, we must use a very large
telescope, for the smaller the angle the farther apart must be the
two holes over the object-glass (or the mirror, in case a reflecting
telescope is employed). In fact, when the holes are moved apart to
the full aperture of the 100-inch Hooker telescope, the interference
fringes are still visible even with the star Betelgeuse, though its
angular diameter is perhaps as great as that of any other star.
Thus, we must build an attachment for the telescope, so arranged
as to permit us to move the openings still farther apart.

[Illustration: Fig. 23. Diagram showing outline of the 100-inch
Hooker telescope, and path of the two pencils of light from a star
when under observation with the 20-foot Michelson interferometer.

A photograph of the interferometer is shown in Fig. 24.]

THE 20-FOOT INSTRUMENT

The 20-foot interferometer designed by Messrs. Michelson and Pease,
and constructed in the Mount Wilson Observatory instrument-shop,
is shown in the diagram (Fig. 23) and in a photograph of the upper
end of the skeleton tube of the telescope (Fig. 24). The light from
the star is received by two flat mirrors (Ml, M4) which project
beyond the tube and can be moved apart along the supporting arm.
These take the place of the two holes over the object-glass in
our experiment. From these mirrors the light is reflected to a
second pair of flat mirrors (M2, M3), which send it toward the
100-inch concave mirror (M5) at the bottom of the telescope tube.
After this the course of the light is exactly as it would be if
the mirrors M2, M3 were replaced by two holes over the 100-inch
mirror. It is reflected to the convex mirror (M6), then back in
a less rapidly convergent beam toward the large mirror. Before
reaching it the light is caught by the plane mirror (M7) and reflected
through an opening at the side of the telescope tube to the eye-piece
E. Here the fringes are observed with a magnification ranging from
1,500 to 3,000 diameters.

[Illustration: Fig. 24. Twenty-foot Michelson interferometer for
measuring star diameters, attached to upper end of the skeleton
tube of the 100-inch Hooker telescope.

The path of the two pencils of light from the star is shown in
Fig. 23. For a photograph of the entire telescope, see Fig. 4.]

In the practical application of this method to the measurement of
star diameters, the chief problem was whether the atmosphere would
be quiet enough to permit sharp interference fringes to be produced
with light-pencils more than 100 inches apart. After successful
preliminary tests with the 40-inch refracting telescope of the
Yerkes Observatory, Professor Michelson made the first attempt
to see the fringes with the 60-inch and 100-inch reflectors on
Mount Wilson in September, 1919. He was surprised and delighted to
find that the fringes were perfectly sharp and distinct with the
full aperture of both these instruments. Doctor Anderson, of the
observatory staff, then devised a special form of interferometer
for the measurement of close double stars, and applied it with
the 100-inch telescope to the measurement of the orbital motion
of the close components of Capella, with results of extraordinary
accuracy, far beyond anything attainable by previous methods. The
success of this work strongly encouraged the more ambitious project
of measuring the diameter of a star, and the 20-foot interferometer
was built for this purpose.

The difficult and delicate problem of adjusting the mirrors of
this instrument with the necessary extreme accuracy was solved by
Professor Michelson during his visit to Mount Wilson in the summer
of 1920, and with the assistance of Mr. Pease, of the observatory
staff, interference fringes were observed in the case of certain
stars when the mirrors were as much as 18 feet apart. All was thus
in readiness for a decisive test as soon as a suitable star presented
itself.

THE GIANT BETELGEUSE

Russell, Shapley, and Eddington had pointed out Betelgeuse (Arabic
for "the giant's shoulder"), the bright red star in the constellation
of Orion (Fig. 25), as the most favorable of all stars for measurement,
and the last-named had given its angular diameter as 0.051 of a
second of arc. This deduction from theory appeared in his recent
presidential address before the British Association for the Advancement
of Science, in which Professor Eddington remarked: "Probably the
greatest need of stellar astronomy at the present day, in order
to make sure that our theoretical deductions are starting on the
right lines, is some means of measuring the apparent angular diameter
of stars." He then referred to the work already in progress on
Mount Wilson, but anticipated "that atmospheric disturbance will
ultimately set the limit to what can be accomplished."

[Illustration: Fig. 25. The giant Betelgeuse (within the circle),
familiar as the conspicuous red star in the right shoulder of Orion
(Hubble).

Measures with the interferometer show its angular diameter to be
0.047 of a second of arc, corresponding to a linear diameter of
215,000,000 miles, if the best available determination of its distance
can be relied upon. This determination shows Betelgeuse to be 160
light-years from the earth. Light travels at the rate of 186,000
miles per second, and yet spends 160 years on its journey to us
from this star.]

On December 13, 1920, Mr. Pease successfully measured the diameter
of Betelgeuse with the 20-foot interferometer. As the outer mirrors
were separated the interference fringes gradually became less distinct,
as theory requires, and as Doctor Merrill had previously seen when
observing Betelgeuse with the interferometer used for Capella. At
a separation of 10 feet the fringes disappeared completely, giving
the data required for calculating the diameter of the star. To
test the perfection of the adjustment, the telescope was turned to
other stars, of smaller angular diameter, which showed the fringes
with perfect clearness. Turning back to Betelgeuse, they were seen
beyond doubt to be absent. Assuming the mean wave-length of the
light of this star to be 5750/10000000 of a millimetre, its angular
diameter comes out 0.047 of a second of arc, thus falling between
the values--0.051 and 0.031 of a second--predicted by Eddington and
Russell from slightly different assumptions. Subsequent corrections
and repeated measurement will change Mr. Pease's result somewhat,
but it is almost certainly within 10 or 15 per cent of the truth.
We may therefore conclude that the angular diameter of Betelgeuse
is very nearly the same as that of a ball one inch in diameter,
seen at a distance of seventy miles.

[Illustration: Fig. 26. Arcturus (within the white circle), known
to the Arabs as the "Lance Bearer," and to the Chinese as the "Great
Horn" or the "Palace of the Emperors" (Hubble).

Its angular diameter, measured at Mount Wilson by Pease with the
20-foot Michelson interferometer on April 15, 1921, is 0.022 of a
second, in close agreement with Russell's predicted value of 0.019
of a second. The mean parallax of Arcturus, based upon several
determinations, is 0.095 of a second, corresponding to a distance of
34 light-years. The linear diameter, computed from Pease's measure
and this value of the distance is about 21 million miles.]

But this represents only the angle subtended by the star's disk.
To learn its linear diameter, we must know its distance. Four
determinations of the parallax, which determines the distance,
have been made. Elkin, with the Yale heliometer, obtained 0.032
of a second of arc. Schlesinger, from photographs taken with the
30-inch Allegheny refractor, derived 0.016. Adams, by his spectroscopic
method applied with the 60-inch Mount Wilson reflector, obtained
0.012. Lee's recent value, secured photographically with the 40-inch
Yerkes refractor, is 0.022. The heliometer parallax is doubtless
less reliable than the photographic ones, and Doctor Adams states
that the spectral type and luminosity of Betelgeuse make his value
less certain than in the case of most other stars. If we take a
(weighted) mean value of 0.020 of a second, we shall probably not
be far from the truth. This parallax represents the angle subtended
by the radius of the earth's orbit (93,000,000 miles) at the distance
of Betelgeuse. By comparing it with 0.047, the angular diameter of
the star, we see that the linear diameter is about two and one-third
times as great as the distance from the earth to the sun, or
approximately 215,000,000 miles. Thus, if this measure of its distance
is not considerably in error, Betelgeuse would nearly fill the
orbit of Mars. All methods of determining the distances of the
stars are subject to uncertainty, however, and subsequent measures
may reduce this figure very appreciably. But there can be no doubt
that the diameter of Betelgeuse exceeds 100,000,000 miles, and
it is probably much greater.

The extremely small angle subtended by this enormous disk is explained
by the great distance of the star, which is about 160 light-years.
That is to say, light travelling at the rate of 186,000 miles per
second spends 160 years in crossing the space that lies between
us and Betelgeuse, whose tremendous proportions therefore seem
so minute even in the most powerful telescopes.

STELLAR EVOLUTION

This actual measure of the diameter of Betelgeuse supplies a new
and striking test of Russell's and Hertzsprung's theory of dwarf
and giant stars. Just before the war Russell showed that our old
methods of classifying the stars according to their spectra must
be radically changed. Stars in an early stage of their life history
may be regarded as diffuse gaseous masses, enormously larger than
our sun, and at a much lower temperature. Their density must be
very low, and their state that of a perfect gas. These are the
"giants." In the slow process of time they contract through constant
loss of heat by radiation. But, despite this loss, the heat produced
by contraction and from other sources (see p. 82) causes their
temperature to rise, while their color changes from red to bluish
white. The process of shrinkage and rise of temperature goes on so
long as they remain in the state of a perfect gas. But as soon as
contraction has increased the density of the gas beyond a certain
point the cycle reverses and the temperature begins to fall. The
bluish-white light of the star turns yellowish, and we enter the
dwarf stage, of which our own sun is a representative. The density
increases, surpassing that of water in the case of the sun, and
going far beyond this point in later stages. In the lapse of millions
of years a reddish hue appears, finally turning to deep red. The
falling temperature permits the chemical elements, existing in a
gaseous state in the outer atmosphere of the star, to unite into
compounds, which are rendered conspicuous by their characteristic
bands in the spectrum. Finally comes extinction of light, as the
star approaches its ultimate state of a cold and solid globe.

[Illustration: Fig. 27. The giant star Antares (within the white
circle), notable for its red color in the constellation Scorpio,
and named by the Greeks "A Rival of Mars" (Hubble).

The distance of Antares, though not very accurately known, is probably
not far from 350 light-years. Its angular diameter of 0.040 of a
second would thus correspond to a linear diameter of about 400
million miles.]

We may thus form a new picture of the two branches of the temperature
curve, long since suggested by Lockyer, on very different grounds, as
the outline of stellar life. On the ascending side are the giants,
of vast dimensions and more diffuse than the air we breathe. There
are good reasons for believing that the mass of Betelgeuse cannot
be more than ten times that of the sun, while its volume is at
least a million times as great and may exceed eight million times
the sun's volume. Therefore, its average density must be like that
of an attenuated gas in an electric vacuum tube. Three-quarters
of the naked-eye stars are in the giant stage, which comprises
such familiar objects as Betelgeuse, Antares, and Aldebaran, but
most of them are much denser than these greatly inflated bodies.
The pinnacle is reached in the intensely hot white stars of the
helium class, in whose spectra the lines of this gas are very
conspicuous. The density of these stars is perhaps one-tenth that
of the sun. Sirius, also very hot, is nearly twice as dense. Then
comes the cooling stage, characterized, as already remarked, by
increasing density, and also by increasing chemical complexity
resulting from falling temperature. This life cycle is probably
not followed by all stars, but it may hold true for millions of
them.

The existence of giant and dwarf stars has been fully proved by
the remarkable work of Adams and his associates on Mount Wilson,
where his method of determining a star's distance and intrinsic
luminosity by spectroscopic observations has already been applied
to 2,000 stars. Discussion of the results leads at once to the
recognition of the two great classes of giants and dwarfs. Now
comes the work of Michelson and Pease to cap the climax, giving us
the actual diameter of a typical giant star, in close agreement with
predictions based upon theory. From this diameter we may conclude that
the density of Betelgeuse is extremely low, in harmony with Russell's
theory, which is further supported by spectroscopic analysis of
the star's light, revealing evidence of the comparatively low
temperature called for by the theory at this early stage of stellar
existence.

TWO OTHER GIANTS

The diameter of Arcturus was successfully measured by Mr. Pease
at Mount Wilson on April 15. As the mirrors of the interferometer
were moved apart, the fringes gradually decreased in visibility
until they finally disappeared at a mirror separation of 19.6 feet.
Adopting a mean wave-length of 5600/10000000 of a millimetre for
the light of Arcturus, this gives a value of 0.022 of a second of
arc for the angular diameter of the star. If we use a mean value
of 0.095 of a second for the parallax, the corresponding linear
diameter comes out 21,000,000 miles. The angular diameter, as in
the case of Betelgeuse, is in remarkably close agreement with the
diameter predicted from theory. Antares, the third star measured
by Mr. Pease, is the largest of all. If it is actually a member of
the Scorpius-Centaurus group, as we have strong reason to believe,
it is fully 350 light-years from the earth, and its diameter is
about 400,000,000 miles.

[Illustration: Fig. 28. Diameters of the Sun, Arcturus, Betelgeuse,
and Antares compared with the orbit of Mars.

Sun, diameter, 865,000 miles.

Arcturus, diameter, 21,000,000 miles.

Betelgeuse, diameter, 215,000,000 miles.

Antares, diameter, 400,000,000 miles.]

It now remains to make further measures of Betelgeuse, especially
because its marked changes in brightness suggest possible variations
in diameter. We must also apply the interferometer method to stars
of the various spectral types, in order to afford a sure basis for
future studies of stellar evolution. Unfortunately, only a few
giant stars are certain to fall within the range of our present
instrument. An interferometer of 70-feet aperture would be needed
to measure Sirius accurately, and one of twice this size to deal
with less brilliant white stars. A 100-foot instrument, if feasible
to build, would permit objects representing most of the chief stages
of stellar development to be measured, thus contributing in the
highest degree to the progress of our knowledge of the life history
of the stars. Fortunately, though the mechanical difficulties are
great, the optical problem is insignificant, and the cost of the
entire apparatus, though necessarily high, would be only a small
fraction of that of a telescope of corresponding aperture, if such
could be built. A 100-foot interferometer might be designed in
many different forms, and one of these may ultimately be found
to be within the range of possibility. Meanwhile the 20-foot
interferometer has been improved so materially that it now promises
to yield approximate measures of stars at first supposed to be
beyond its capacity.

[Illustration: Fig. 29. Aldebaran, the "leader" (of the Pleiades),
was also known to the Arabs as "The Eye of the Bull," "The Heart
of the Bull," and "The Great Camel" (Hubble).

Like Betelgeuse and Antares, it is notable for its red color, which
accounts for the fact that its image on this photograph is hardly
more conspicuous than the images of stars which are actually much
fainter but contain a larger proportion of blue light, to which
the photographic plates here employed are more sensitive than to
red or yellow. Aldebaran is about 50 light-years from the earth.
Interferometer measures, now in progress on Mount Wilson, indicate
that its angular diameter is about 0.020 of a second.]

While the theory of dwarf and giant stars and the measurements just
described afford no direct evidence bearing on Laplace's explanation
of the formation of planets, they show that stars exist which are
comparable in diameter with our solar system, and suggest that the
sun must have shrunk from vast dimensions. The mode of formation
of systems like our own, and of other systems numerously illustrated
in the heavens, is one of the most fascinating problems of astronomy.
Much light has been thrown on it by recent investigations, rendered
possible by the development of new and powerful instruments and by
advances in physics of the most fundamental character. All the
evidence confirms the existence of dwarf and giant stars, but much
work must be done before the entire course of stellar evolution
can be explained.




CHAPTER III

COSMIC CRUCIBLES

"Shelter during Raids," marking the entrance to underground passages,
was a sign of common occurrence and sinister suggestion throughout
London during the war. With characteristic ingenuity and craftiness,
ostensibly for purposes of peace but with bomb-carrying capacity
as a prime specification, the Zeppelin had been developed by the
Germans to a point where it seriously threatened both London and
Paris. Searchlights, range-finders, and anti-aircraft guns, surpassed
by the daring ventures of British and French airmen, would have
served but little against the night invader except for its one
fatal defect--the inflammable nature of the hydrogen gas that kept
it aloft. A single explosive bullet served to transform a Zeppelin
into a heap of scorched and twisted metal. This characteristic
of hydrogen caused the failure of the Zeppelin raids.

Had the war lasted a few months longer, however, the work of American
scientists would have made our counter-attack in the air a formidable
one. At the signing of the armistice hundreds of cylinders of compressed
helium lay at the docks ready for shipment abroad. Extracted from
the natural gas of Texas wells by new and ingenious processes,
this substitute for hydrogen, almost as light and absolutely
uninflammable, produced in quantities of millions of cubic feet,
would have made the dirigibles of the Allies masters of the air. The
special properties of this remarkable gas, previously obtainable only
in minute quantities, would have sufficed to reverse the situation.

SOLAR HELIUM

Helium, as its name implies, is of solar origin. In 1868, when
Lockyer first directed his spectroscope to the great flames or
prominences that rise thousands of miles, sometimes hundreds of
thousands, above the surface of the sun, he instantly identified
the characteristic red and blue radiations of hydrogen. In the
yellow, close to the position of the well-known double line of
sodium, but not quite coincident with it, he detected a new line,
of great brilliancy, extending to the highest levels. Its similarity
in this respect with the lines of hydrogen led him to recognize
the existence of a new and very light gas, unknown to terrestrial
chemistry.

Many years passed before any chemical laboratory on earth was able
to match this product of the great laboratory of the sun. In 1896
Ramsay at last succeeded in separating helium, recognized by the same
yellow line in its spectrum, in minute quantities from the mineral
uraninite. Once available for study under electrical excitation in
vacuum tubes, helium was found to have many other lines in its
spectrum, which have been identified in the spectra of solar
prominences, gaseous nebulæ, and hot stars. Indeed, there is a
stellar class known as helium stars, because of the dominance of
this gas in their atmospheres.

[Illustration: Fig. 30. Solar prominences, photographed with the
spectroheliograph without an eclipse (Ellerman).

In these luminous gaseous clouds, which sometimes rise to elevations
exceeding half the sun's diameter, the new gas helium was discovered
by Lockyer in 1868. Helium was not found on the earth until 1896.
Since then it has been shown to be a prominent constituent of nebulæ
and hot stars.]

The chief importance of helium lies in the clue it has afforded to
the constitution of matter and the transmutation of the elements.
Radium and other radioactive substances, such as uranium, spontaneously
emit negatively charged particles of extremely small mass (electrons),
and also positively charged particles of much greater mass, known
as alpha particles. Rutherford and Geiger actually succeeded in
counting the number of alpha particles emitted per second by a
known mass of radium, and showed that these were charged helium
atoms.

To discuss more at length the extraordinary characteristics of
helium, which plays so large a part in celestial affairs, would
take us too far afield. Let us therefore pass to another case in
which a fundamental discovery, this time in physics, was first
foreshadowed by astronomical observation.

SUN-SPOTS AS MAGNETS

No archæologist, whether Young or Champollion deciphering the Rosetta
Stone, or Rawlinson copying the cuneiform inscription on the cliff
of Behistun, was ever faced by a more fascinating problem than that
which confronts the solar physicist engaged in the interpretation
of the hieroglyphic lines of sun-spot spectra. The colossal whirling
storms that constitute sun-spots, so vast that the earth would make
but a moment's scant mouthful for them, differ materially from
the general light of the sun when examined with the spectroscope.
Observing them visually many years ago, the late Professor Young,
of Princeton, found among their complex features a number of double
lines which he naturally attributed, in harmony with the physical
knowledge of the time, to the effect of "reversal" by superposed
layers of vapors of different density and temperature. What he
actually saw, however, as was proved at the Mount Wilson Observatory
in 1908, was the effect of a powerful magnetic field on radiation,
now known as the Zeeman effect.

[Illustration: Fig. 31. The 150-foot tower telescope of the Mount
Wilson Observatory.

An image of the sun about 16 inches in diameter is formed in the
laboratory at the base of the tower. Below this, in a well extending
80 feet into the earth, is the powerful spectroscope with which
the magnetic fields in sun-spots and the general magnetic field
of the sun are studied.]

Faraday was the first to detect the influence of magnetism on light.
Between the poles of a large electromagnet, powerful for those
days (1845), he placed a block of very dense glass. The plane of
polarization of a beam of light, which passed unaffected through
the glass before the switch was closed, was seen to rotate when the
magnetic field was produced by the flow of the current. A similar
rotation is now familiar in the well-known tests of sugars--lævulose
and dextrose--which rotate plane-polarized light to left and right,
respectively.

But in this first discovery of a relationship between light and
magnetism Faraday had not taken the more important step that he
coveted--to determine whether the vibration period of a light-emitting
particle is subject to change in a magnetic field. He attempted
this in 1862--the last experiment of his life. A sodium flame was
placed between the poles of a magnet, and the yellow lines were
watched in a spectroscope when the magnet was excited. No change
could be detected, and none was found by subsequent investigators
until Zeeman, of Leiden, with more powerful instruments made his
famous discovery, the twenty-fifth anniversary of which has recently
been celebrated.

[Illustration: Fig. 32. Pasadena Laboratory of the Mount Wilson
Observatory.

Showing the large magnet (on the left) and the spectroscopes used
for the study of the effect of magnetism on radiation. A single line
in the spectrum is split by the magnetic field into from three to
twenty-one components, as illustrated in Fig. 34. The corresponding
lines in the spectra of sun-spots are split up in precisely the
same way, thus indicating the presence of powerful magnetic fields
in the sun.]

His method of procedure was similar to Faraday's, but his magnet and
spectroscope were much more powerful, and a theory due to Lorentz,
predicting the nature of the change to be expected, was available
as a check on his results. When the current was applied the lines
were seen to widen. In a still more powerful magnetic field each
of them split into two components (when the observation was made
along the lines of force), and the light of the components of each
line was found to be circularly polarized in opposite directions.
Strictly in harmony with Lorentz's theory, this splitting and
polarization proved the presence in the luminous vapor of exactly such
negatively charged electrons as had been indicated there previously
by very different experimental methods.

In 1908 great cyclonic storms, or vortices, were discovered at
the Mount Wilson Observatory centring in sun-spots. Such whirling
masses of hot vapors, inferred from Sir Joseph Thomson's results
to contain electrically charged particles, should give rise to a
magnetic field. This hypothesis at once suggested that the double
lines observed by Young might really represent the Zeeman effect.
The test was made, and all the characteristic phenomena of radiation
in a magnetic field were found.

Thus a great physical experiment is constantly being performed
for us in the sun. Every large sunspot contains a magnetic field
covering many thousands of square miles, within which the spectrum
lines of iron, manganese, chromium, titanium, vanadium, calcium,
and other metallic vapors are so powerfully affected that their
widening and splitting can be seen with telescopes and spectroscopes
of moderate size.

THE TOWER TELESCOPE

Both of these illustrations show how the physicist and chemist,
when adequately armed for astronomical attack, can take advantage
in their studies of the stupendous processes visible in cosmic
crucibles, heated to high temperatures and influenced, as in the
case of sun-spots, by intense magnetic fields. Certain modern
instruments, like the 60-foot and 150-foot tower telescopes on
Mount Wilson, are especially designed for observing the course
of these experiments. The second of these telescopes produces at
a fixed point in a laboratory an image of the sun about 16 inches
in diameter, thus enlarging the sun-spots to such a scale that
the magnetic phenomena of their various parts can be separately
studied. This analysis is accomplished with a spectroscope 80 feet
in length, mounted in a subterranean chamber beneath the tower. The
varied results of such investigations cannot be described here.
Only one of them may be mentioned--the discovery that the entire sun,
rotating on its axis, is a great magnet. Hence we may reasonably
infer that every star, and probably every planet, is also a magnet,
as the earth has been known to be since the days of Gilbert's "De
Magnete." Here lies one of the best clues for the physicist who
seeks the cause of magnetism, and attempts to produce it, as Barnett
has recently succeeded in doing, by rapidly whirling masses of
metal in the laboratory.

[Illustration: Fig. 33. Sun-spot vortex in the upper hydrogen
atmosphere. (Benioff).

Photographed with the spectroheliograph. The electric vortex that
causes the magnetic field of the spot lies at a lower level, and
is not shown by such photographs.]

Perhaps a word of caution should be interpolated at this point.
Solar magnetism in no wise accounts for the sun's gravitational
power. Indeed, its attraction cannot be felt by the most delicate
instruments at the distance of the earth, and would still be unknown
were it not for the influence of magnetism on light.

Auroras, magnetic storms, and such electric currents as those that
recently deranged several Atlantic cables are due, not to the magnetism
of the sun or its spots, but probably to streams of electrons, shot
out from highly disturbed areas of the solar surface surrounding
great sun-spots, traversing ninety-three million miles of the ether
of space, and penetrating deep into the earth's atmosphere. These
striking phenomena lead us into another chapter of physics, which
limitations of space forbid us to pursue.

STELLAR CHEMISTRY

Let us turn again to chemistry, and see where experiments performed
in cosmic laboratories can serve as a guide to the investigator.
A spinning solar tornado, incomparably greater in scale than the
devastating whirlwinds that so often cut narrow paths of destruction
through town and country in the Middle West, gradually gives rise
to a sun-spot. The expansion produced by the centrifugal force at
the centre of the storm cools the intensely hot gases of the solar
atmosphere to a point where chemical union can occur. Titanium
and oxygen, too hot to combine in most regions of the sun, join
to form the vapor of titanium oxide, characterized in the sunspot
spectrum by fluted bands, made up of hundreds of regularly spaced
lines. Similarly magnesium and hydrogen combine as magnesium hydride
and calcium and hydrogen form calcium hydride. None of these compounds,
stable at the high temperatures of sun-spots, has been much studied
in the laboratory. The regions in which they exist, though cooler
than the general atmosphere of the sun, are at temperatures of
several thousand degrees, attained in our laboratories only with
the aid of such devices as powerful electric furnaces.

[Illustration: Fig. 34. Splitting of spectrum lines by a magnetic
field (Babcock).

The upper and lower strips show lines in the spectrum of chromium,
observed without a magnetic field. When subjected to the influence
of magnetism, these single lines are split into several components.
Thus the first line on the right is resolved by the field into
three components, one of which (plane polarized) appears in the
second strip, while the other two, which are polarized in a plane
at right angles to that of the middle component, are shown on the
third strip. The next line is split by the magnetic field into
twelve components, four of which appear in the second strip and
eight in the third. The magnetic fields in sun-spots affect these
lines in precisely the same way.]

It is interesting to follow our line of reasoning to the stars,
which differ widely in temperature at various stages in their
life-cycle.[*] A sun-spot is a solar tornado, wherein the intensely
hot solar vapors are cooled by expansion, giving rise to the compounds
already named. A red star, in Russell's scheme of stellar evolution,
is a cooler sun, vast in volume and far more tenuous than atmospheric
air when in the initial period of the "giant" stage, but compressed
and denser than water in the "dwarf" stage, into which our sun has
already entered as it gradually approaches the last phases of its
existence. Therefore we should find, throughout the entire atmosphere
of such stars, some of the same compounds that are produced within
the comparatively small limits of a sun-spot. This, of course,
on the correct assumption that sun and stars are made of the same
substances. Fowler has already identified the bands of titanium
oxide in such red stars as the giant Betelgeuse, and in others
of its class. It is safe to predict that an interesting chapter
in the chemistry of the future will be based upon the study of
such compounds, both in the laboratory and under the progressive
temperature conditions afforded by the countless stellar "giants"
and "dwarfs" that precede and follow the solar state.

[Footnote *: See Chapter II.]

[Illustration: Fig. 35. Electric furnace in the Pasadena laboratory
of the Mount Wilson Observatory.

With which the chemical phenomena observed in sun-spots and red
stars are experimentally imitated.]

ASTROPHYSICAL LABORATORIES

It is precisely in this long sequence of physical and chemical
changes that the astrophysicist and the astrochemist can find the
means of pushing home their attack. It is true, of course, that
the laboratory investigator has a great advantage in his ability
to control his experiments, and to vary their progress at will.
But by judicious use of the transcendental temperatures, far out
ranging those of his furnaces, and extreme conditions, which he
can only partially imitate, afforded by the sun, stars, and nebulæ,
he may greatly widen the range of his inquiries. The sequence of
phenomena seen during the growth of a sun-spot, or the observation
of spots of different sizes, and the long series of successive
steps that mark the rise and decay of stellar life, resemble the
changes that the experimenter brings about as he increases and
diminishes the current in the coils of his magnet or raises and
lowers the temperature of his electric furnace, examining from
time to time the spectrum of the glowing vapors, and noting the
changes shown by the varying appearance of their lines.

[Illustration: Fig. 36. Titanium oxide in red stars.

The upper spectrum is that of titanium in the flame of the electric
arc, where its combination with oxygen gives rise to the bands of
titanium oxide (Fowler). The lower strip shows the spectrum of
the red star Mira (Omicron Ceti), as drawn by Cortie at Stonyhurst.
The bands of titanium oxide are clearly present in the star.]

[Illustration: Fig. 37. Titanium oxide in sun-spots.

The upper strip shows a portion of the spectrum of a sun-spot
(Ellerman); the lower one the corresponding region of the spectrum
of titanium oxide (King). The fluted bands of the oxide spectrum
are easily identified in the spot, where they indicate that titanium
and oxygen, too hot to combine in the solar atmosphere, unite in the
spot because of the cooling produced by expansion in the vortex.]

Astronomical observations of this character, it should be noted, are
most effective when constantly tested and interpreted by laboratory
experiment. Indeed, a modern astrophysical observatory should be
equipped like a great physical laboratory, provided on the one hand
with telescopes and accessory apparatus of the greatest attainable
power, and on the other with every device known to the investigator
of radiation and the related physical and chemical phenomena. Its
telescopes, especially designed with the aims of the physicist and
chemist in view, bring images of sun, stars, nebulæ, and other
heavenly bodies within the reach of powerful spectroscopes, sensitive
bolometers and thermopiles, and the long array of other appliances
available for the measurement and analysis of radiation. Its electric
furnaces, arcs, sparks, and vacuum tubes, its apparatus for increasing
and decreasing pressure, varying chemical conditions, and subjecting
luminous gases and vapors to the influence of electric and magnetic
fields, provide the means of imitating celestial phenomena, and of
repeating and interpreting the experiments observed at the telescope.
And the advantage thus derived, as we have seen, is not confined
to the astronomer, who has often been able, by making fundamental
physical and chemical discoveries, to repay his debt to the physicist
and chemist for the apparatus and methods which he owes to them.

NEWTON AND EINSTEIN

Take, for another example, the greatest law of physics--Newton's
law of gravitation. Huge balls of lead, as used by Cavendish, produce
by their gravitational effect a minute rotation of a delicately
suspended bar, carrying smaller balls at its extremities. But no
such feeble means sufficed for Newton's purpose. To prove the law
of gravitation he had recourse to the tremendous pull on the moon
of the entire mass of the earth, and then extended his researches
to the mutual attractions of all the bodies of the solar system.
Later Herschel applied this law to the suns which constitute double
stars, and to-day Adams observes from Mount Wilson stars falling
with great velocity toward the centre of the galactic system under
the combined pull of the millions of objects that compose it. Thus
full advantage has been taken of the possibility of utilizing the
great masses of the heavenly bodies for the discovery and application
of a law of physics and its reciprocal use in explaining celestial
motions.

[Illustration: Fig. 38. The Cavendish experiment.

Two lead balls, each two inches in diameter, are attached to the
ends of a torsion rod six feet long, which is suspended by a fine
wire. The experiment consists in measuring the rotation of the
suspended system, caused by the gravitational attraction of two
lead spheres, each twelve inches in diameter, acting on the two
small lead balls.]

Or consider the Einstein theory of relativity, the truth or falsity
of which is no less fundamental to physics. Its inception sprang from
the Michelson-Morley experiment, made in a laboratory in Cleveland,
which showed that motion of the earth through the ether of space could
not be detected. All of the three chief tests of Einstein's general
theory are astronomical--because of the great masses required to
produce the minute effects predicted: the motion of the perihelion
of Mercury, the deflection of the light of a star by the attraction
of the sun, and the shift of the lines of the solar spectrum toward
the red--questions not yet completely answered.

But it is in the study of the constitution of matter and the evolution
of the elements, the deepest and most critical problem of physics
and chemistry, that the extremes of pressure and temperature in the
heavenly bodies, and the prevalence of other physical conditions not
yet successfully imitated on earth, promise the greatest progress.
It fortunately happens that astrophysical research is now at the
very apex of its development, founded as it is upon many centuries
of astronomical investigation, rejuvenated by the introduction
into the observatory of all the modern devices of the physicist,
and strengthened with instruments of truly extraordinary range
and power. These instruments bring within reach experiments that
are in progress on some minute region of the sun's disk, or in
some star too distant even to be glimpsed with ordinary telescopes.
Indeed, the huge astronomical lenses and mirrors now available
serve for these remote light-sources exactly the purpose of the
lens or mirror employed by the physicist to project upon the slit
of his spectroscope the image of a spark or arc or vacuum tube
within which atoms and molecules are exposed to the influence of
the electric discharge. The physicist has the advantage of complete
control over the experimental conditions, while the astrophysicist
must observe and interpret the experiments performed for him in
remote laboratories. In actual practice, the two classes of work
must be done in the closest conjunction, if adequate utilization
is to be made of either. And this is only natural, for the trend
of recent research has made clear the fact that one of the three
greatest problems of modern astronomy and astrophysics, ranking
with the structure of the universe and the evolution of celestial
bodies, is the constitution of matter. Let us see why this is so.

TRANSMUTATION OF THE ELEMENTS

The dream of the alchemist was to transmute one element into another,
with the prime object of producing gold. Such transmutation has been
actually accomplished within the last few years, but the process
is invariably one of disintegration--the more complex elements
being broken up into simpler constituents. Much remains to be done
in this same direction; and here the stars and nebulæ, which show
the spectra of the elements under a great variety of conditions,
should help to point the way. The progressive changes in spectra,
from the exclusive indications of the simple elements hydrogen,
helium, nitrogen, possibly carbon, and the terrestrially unknown
gas nebulium in the gaseous nebulæ, to the long list of familiar
substances, including several chemical compounds, in the red stars,
may prove to be fundamentally significant when adequately studied
from the standpoint of the investigator of atomic structure. The
existing evidence seems to favor the view, recently expressed by
Saha, that many of these differences are due to varying degrees
of ionization, the outer electrons of the atoms being split off
by high temperature or electrical excitation. It is even possible
that cosmic crucibles, unrivalled by terrestrial ones, may help
materially to reveal the secret of the formation of complex elements
from simpler ones. Physicists now believe that all of the elements are
compounded of hydrogen atoms, bound together by negative electrons.
Thus helium is made up of four hydrogen atoms, yet the atomic weight
of helium (4) is less than four times that of hydrogen (1.008).
The difference may represent the mass of the electrical energy
released when the transmutation occurred.

[Illustration: Fig. 39. The Trifid Nebula in Sagittarius (Ritchey).

The gas "nebulium," not yet found on the earth, is the most
characteristic constituent of irregular nebulæ. Nebulium is recognized
by two green lines in its spectrum, which cause the green color of
nebulæ of the gaseous type.]

Eddington has speculated in a most interesting way on this possible
source of stellar heat in his recent presidential address before the
British Association for the Advancement of Science (see _Nature_,
September 2, 1920). He points out that the old contraction hypothesis,
according to which the source of solar and stellar heat was supposed
to reside in the slow condensation of a radiating mass of gas under
the action of gravity, is wholly inadequate to explain the observed
phenomena. If the old view were correct, the earlier history of
a star, from the giant stage of a cool and diaphanous gas to the
period of highest temperature, would be run through within eighty
thousand years, whereas we have the best of evidence that many
thousands of centuries would not suffice. Some other source of energy
is imperatively needed. If 5 per cent of a star's mass consists
originally of hydrogen atoms, which gradually combine in the slow
process of time to form more complex elements, the total heat thus
liberated would more than suffice to account for all demands, and
it would be unnecessary to assume the existence of any other source
of heat.

[Illustration: Fig. 40. Spiral nebula in Ursa Major (Ritchey).

Luminous matter, in every variety of physical and chemical state,
is available for study in the most diverse celestial objects, from
the spiral and irregular nebulæ through all the types of stars.
Doctor van Maanen's measures of the Mount Wilson photographs indicate
outward motion along the arms of spiral nebulæ, while the spectroscope
shows them to be whirling at enormous velocities.]

COSMIC PRESSURES

This, it may fairly be said, is very speculative, but the fact
remains that celestial bodies appear to be the only places in which
the complex elements may be in actual process of formation from their
known source--hydrogen. At least we may see what a vast variety
of physical conditions these cosmic crucibles afford. At one end of
the scale we have the excessively tenuous nebulæ, the luminosity of
which, mysterious in its origin, resembles the electric glow in our
vacuum tubes. Here we can detect only the lightest and simplest of
the elements. In the giant stars, also extremely tenuous (the density
of Betelgeuse can hardly exceed one-thousandth of an atmosphere) we
observe the spectra of iron, manganese, titanium, calcium, chromium,
magnesium, vanadium, and sodium, in addition to titanium oxide.
The outer part of these bodies, from which light reaches us, must
therefore be at a temperature of only a few thousand degrees, but
vastly higher temperatures must prevail at their centres. In passing
up the temperature curve more and more elements appear, the surface
temperature rises, and the internal temperature may reach millions
of degrees. At the same time the pressure within must also rise,
reaching enormous figures in the last stages of stellar life. Cook
has calculated that the pressure at the centre of the earth is
between 4,000 and 10,000 tons per square inch, and this must be
only a very small fraction of that attained within larger celestial
bodies. Jeans has computed the pressure at the centre of two colliding
stars as they strike and flatten, and finds it may be of the order
of 1,000,000,000 tons per square inch--sufficient, if their diameter
be equal to that of the sun--to vaporize them 100,000 times over.

Compare these pressures with the highest that can be produced on
earth. If the German gun that bombarded Paris were loaded with a
solid steel projectile of suitable dimensions, a muzzle velocity
of 6,000 feet per second could be reached. Suppose this to be fired
into a tapered hole in a great block of steel. The instantaneous
pressure, according to Cook, would be about 7,000 tons per square
inch, only 1/150000 of that possible through the collision of the
largest stars.

[Illustration: Fig. 41. Mount San Antonio as seen from Mount Wilson.

Michelson is measuring the velocity of light between stations on
Mount Wilson and Mount San Antonio. Astronomical observations afford
the best means, however, of detecting any possible difference between
the velocities of light of different colors. From studies of variable
stars in the cluster Messier 5 Shapley concludes that if there is
any difference between the velocities of blue and yellow light
in free space it cannot exceed two inches in one second, the time
in which light travels 186,000 miles.]

Finally, we may compare the effects of light pressure on the earth
and stars. Twenty years ago Nichols and Hull succeeded, with the
aid of the most sensitive apparatus, in measuring the minute
displacements produced by the pressure of light. The effect is
so slight, even with the brightest light-sources available, that
great experimental skill is required to measure it. Yet in the
case of some of the larger stars Eddington calculates that one-half
of their mass is supported by radiation pressure, and this against
their enormous gravitational attraction. In fact, if their mass
were as great as ten times that of the sun, the radiation pressure
would so nearly overcome the pull of gravitation that they would
be likely to break up.

But enough has been said to illustrate the wide variety of experimental
devices that stand at our service in the laboratories of the heavens.
Here the physicist and chemist of the future will more and more
frequently supplement their terrestrial apparatus, and find new
clues to the complex problems which the amazing progress of recent
years has already done so much to solve.

PRACTICAL VALUE OF RESEARCHES ON THE CONSTITUTION OF MATTER

The layman has no difficulty in recognizing the practical value
of researches directed toward the improvement of the incandescent
lamp or the increased efficiency of the telephone. He can see the
results in the greatly decreased cost of electric illumination
and the rapid extension of the range of the human voice. But the
very men who have made these advances, those who have succeeded
beyond all expectation in accomplishing the economic purposes in
view, are most emphatic in their insistence upon the importance
of research of a more fundamental character. Thus Vice-President
J. J. Carty, of the American Telephone and Telegraph Company, who
directs its great Department of Development and Research, and Doctor
W. J. Whitney, Director of the Research Laboratory of the General
Electric Company, have repeatedly expressed their indebtedness
to the investigations of the physicist, made with no thought of
immediate practical return. Faraday, studying the laws of electricity,
discovered the principle which rendered the dynamo possible. Maxwell,
Henry, and Hertz, equally unconcerned with material advantage,
made wireless telegraphy practicable. In fact, all truly great
advances are thus derived from fundamental science, and the future
progress of the world will be largely dependent upon the provision
made for scientific research, especially in the fields of physics
and chemistry, which underlie all branches of engineering.

The constitution of matter, therefore, instead of appealing as
a subject to research only to the natural philosopher or to the
general student of science, is a question of the greatest practical
concern. Already the by-products of investigations directed toward
its elucidation have been numerous and useful in the highest degree.
Helium has been already cited; X-rays hardly require mention; radium,
which has so materially aided sufferers from cancer, is still better
known. Wireless telephony and transcontinental telephony with wires
were both rendered possible by studies of the nature of the electric
discharge in vacuum tubes. Thus the "practical man," with his distrust
of "pure" science, need not resent investments made for the purpose
of advancing our knowledge of such fundamental subjects as physics
and chemistry. On the contrary, if true to his name, he should
help to multiply them many fold in the interest of economic and
commercial development.